eb594224   
   On Friday, November 13, 2009 at 7:20:30 AM UTC-8, kreed wrote:   
   > Some of you may be aware of this, especially if you had a number of   
   > CD51's in operation,  but thought I would pass on these observations   
   > on to avoid anyone else being caught.  Note that while these machines   
   > are probably not used on location as much these days as in the past,   
   > many have been possibly been converted to digital, and use the same   
   > amplifier and sound system regardless, so this is still valid for   
   > these people.   
   >    
   > Bought one of these on location from another operator who was sick of   
   > it and the problems, hadn't done any maintenance on it for years, and   
   > didnt have a broad technical knowledge, usually calling me from   
   > location when stuck. As the machines aged, the problems grew more and   
   > more varied, driving him to want out of the business.  The machine had   
   > loads of intermittent faults, but earned well, and the location owner   
   > was a pretty good guy and had been amazingly tolerant of the faults   
   > over some time.   
   >    
   > Usually in cases like this, I would simply replace the entire machine   
   > with one of ours (known good, all mods done) and bring the other home   
   > to go over it and overhaul thoroughly.   
   > In this case, literally nothing other than a CD 51 will fit in the   
   > confined alcove space its in, Its been there since it was bought new,   
   > and it cannot be moved elsewhere due to lack of space,  and CD51 seem   
   > impossible to find in this country, so can't get a replacement. I wish   
   > he has bought a CD 100 instead !   
   >    
   > AS such, I replaced the boards, power supply, checked the harness   
   > connectors and naturally have been ironing out the few remaining   
   > problems quite successfully so far as they have popped up.   
   >    
   > After copping a complaint of "low maximum volume" I checked the volume   
   > control, amplifier, and apart from finding it to be an amp from out of   
   > a CD-100 (7 band graphic equalizer) rather than the 3 band EQ used in   
   > the CD 51, I also found the left channel of the amplifier, power   
   > driver board had many of the transistors burnt up, and charring on the   
   > board under them.   
   > Hadn't seen anything like that before, usually the output devices just   
   > die for no particular reason, blow their associated fuse(s), and the   
   > driver PCB is unharmed.   
   > Usual procedure = Replace transistors (and fuses) check and balance   
   > the idle current if needed and all is well for another few years.   
   >    
   > In this case, I fortunately had a spare, known good CD51 amp that I   
   > had obtained from somewhere in the past, tested it, took it to the   
   > locatio, and put it in.  Substantial increase in volume, great sound   
   > quality, everyone happy.   
   > Location owner really happy, "sounded better than it had in 4 years"   
   > etc. etc.   
   >    
   > At that time that I checked the crossover board for dry joints, shorts   
   > etc as I was very suss about the unusual driver board damage.)   
   >    
   > Found that 4 of the 2W resistors had overheated and gone a bit black,   
   > the ones that divide the signal for the tweeter, but still measured   
   > what looked like sensible values (couldn't read the coloured bands)   
   > ok.  They were getting quite a bit warmer than I would like when   
   > playing loud, but I didn't have replacements on me.   
   >    
   > Noted down to bring 5w units next time and replace them.  Thought it   
   > strange that this would happen.  Listened carefully to tweeters, no   
   > distortion, drops in volume or other signs of distress that may have   
   > meant shorting internally, which would cooking their  resistor,   
   > sounded ok even at high volume.   
   >    
   > Back at the workshop, checking the Amplifier, I found the preamp had   
   > failed also.  One channel had no sound and the other had a faint   
   > "crackle".  After a lot of mucking around, it was discovered that one   
   > set of contacts on the Reed relay (mute) had welded shut, killing the   
   > sound on that channel.  This was totally unexpected and as a result   
   > wasn't easy to diagnose.   
   > This led me to believe that when the transistors failed and burnt up   
   > on the power amp driver board, a high voltage (+ or - 40v rail)   
   > managed to travel back up the line level input to the preamp, welding   
   > the relay contacts.   
   > before the fuse blew.  There was still distortion, even with the relay   
   > fixed.   
   >    
   > Not trusting IC's that I suspect have been subjected to substantial   
   > over voltage, I replaced all the op-amp IC's on the board just to make   
   > sure. They are cheap and easy to do so what the hell -   Worked fine   
   > after this.   
   >    
   > ---------------------------------   
   >    
   > 2 week later, get a call from the location, "it sounds terrible, sound   
   > is broken up".  Thought one of the amp channels had blown an output   
   > transistor and the thing was distorting.  Took spare amp, and   
   > photocopied the pages related to the crossover to help with replacing   
   > the resistors, which I also took.   
   >    
   > Took a spare crossover from out of a CD-100 model, noted it had all 5w   
   > resistors, not 2w carbon ones.   
   >    
   > Got there, machine had no bass whatsoever, but no distortion in the   
   > mid and upper, amp was ok.   
   > sounded absolutely horrid - if you don't believe me, disconnect the   
   > woofers from your juke and listen to how incredibly bad it sounds and   
   > how much "volume" you lose  ;)   
   >    
   > I was getting very concerned, checked the crossover for, bad terminals   
   > etc, but all was ok.   
   >    
   > Pulled out one of the woofers, was open circuit.  Pulled out the   
   > other, same.  Was dumbfounded as to how this could happen, have never   
   > seen this before, not in both woofers.  The last time I saw a problem   
   > like this, the wire that ran from the speaker cone to the terminals   
   > had failed (from metal fatigue?) and would intermittently work/not   
   > work, arcing like crazy when it did work temporarily welding itself   
   > back ?- it was only one speaker not both.   
   >    
   > Pulled out the crossover. 2W carbon Resistors had burnt badly and one   
   > had charred the PCB.  Open circuit, both channels.   
   > Replaced them with 5W units of same value..   
   >    
   > Ended up having to get a pair of woofers and bring them and fit them.   
   > Sounded great.   
   >    
   > After studying the circuit and finally noted on paperwork that I   
   > copied of wiring diagram between crossover and amp  "connect to E6 E6   
   > maximum"   
   > Noticed previous op had put terminals on E7 E7, this is probably what   
   > blew up the old amp and overheated the resistors before it did. Then   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)